Samples taken from a fluidized-bed reactor revealed very low saturation constants for the degradation of acetate (2-12 mg/l) and propionate (<3 mg/l). The higher values for the acetate degradation appear to be caused by mass-transport limitation due to calcium carbonate precipitation within the biofilm. The intrinsic saturation constant is about 3 mg/l, which is significantly lower than previously published values for pure and mixed cultures. The influence of the pH on the saturation constant was investigated in fed-batch experiments. Contrary to the hypothesis that only the undissociated acid is the effective substrate, no significant influence of pH on the saturation constant (given as concentration of total acid) was observed. Batch experiments with n-butyrate revealed hyperbolic progress curves, which might be misinterpreted as a sign of a high saturation constant. However, fed-batch experiments showed that, for n-butyrate degradation, the saturation constant is very low. The isomerisation to isobutyrate and other side-reactions, for which indications were found, influence the progress curve such that an elevated saturation constant will result as an artifact. Thus saturation constants for n-butyrate degradation obtained from batch experiments have to be viewed critically.